Method and system for logging on a mobile unit at a fixed station

ABSTRACT

A method and system for the wire-free transmission of data between a mobile unit and a fixed station in time slots on one of a number of carrier frequencies with fixed station and the mobile unit each respectively include both a device for outputting a predetermined sequence which prescribes the carrier frequencies of the time slots, the carrier frequencies of two successive time slots being different, and an RF module for transmitting the data in time slots, the carrier frequencies of the time slots each being prescribed by the predetermined frequency of the output device.

The present invention relates to a method and system for logging on amobile unit at a fixed station for a transmission of data by radio, inwhich transmission the data is transmitted in time slots on a number ofcarrier frequencies and the carrier frequency is changed from one timeslot to the next in accordance with a predetermined sequence.

DESCRIPTION OF THE PRIOR ART

In the majority of cordless telephones currently available on themarket, it is possible to serve more than one mobile unit from a fixedstation. Often, a cordless telephone system is retrofitted by adding afurther mobile unit to the already existing mobile unit or units. Forthis purpose, the new mobile unit must be logged on in the alreadyexisting cordless telephone system; i.e., in particular, at the fixedstation. Logging on is, therefore, to be understood within the contextof the present Invention to mean that a mobile unit, in particular afurther mobile unit, is logged on in the sense of signing on at thefixed station. As such, once logging on has taken place, the mobile unitcan transmit voice information data to the fixed station and receive itfrom the fixed station as well.

Problems are experienced if a so-called frequency hopping spreadspectrum system is used as air interface and a mobile unit, inparticular a further mobile unit, is to be integrated into such asystem. A frequency hopping spread spectrum system is a system in whicha number of carrier frequencies are available for transmitting data byradio and the carrier frequency used is changed from time to time, forexample, after each time slot or frame of the transmission. Inparticular, in a time division multiplex system (TDMA), the carrierfrequency can be changed after each time slot or time frame of the timedivision multiplex transmission. Such a frequency hopping spreadspectrum system has advantages to the extent that the energy of theentire radio transmission is distributed over all the carrierfrequencies and, thus, one single carrier frequency less is loaded. Thisis particularly important if a generally available frequency band, suchas the 2.4 0 Hz 1524 (Industrial Scientific Medical) band, is used inwhich an upper limit for the maximum energy occurring per carrierfrequency is prescribed in order to keep interference with othersubscribers as low as possible.

A further advantage of the frequency hopping spread spectrum system isthat the provision of a large number of carrier frequencies makes thesystem less susceptible to interference. Furthermore, the protection ofthe system against listening in by third parties is increased, since thethird party does not usually know what the carrier frequency is beingchanged to after a certain time period.

Even if a frequency hopping spread spectrum system has theabovementioned advantages, there is still the problem of synchronizingthe carrier frequencies and, in particular, of changing the carrierfrequencies when logging on a new mobile unit at a fixed station. It is,in fact, a precondition of logging on that the mobile unit to be loggedon is capable of communicating with the fixed station; i.e., canprecisely perform the change of carrier frequency.

WO 95/06377 teaches a method and an system for transmitting datawirelessly between a mobile unit and a fixed station in time slots on anumber of carrier frequencies. In this method and system, the carrierfrequencies of a predetermined time period are changed in accordancewith a predetermined sequence. To accomplish this, the mobile unit andthe fixed station each include a device for outputting the predeterminedsequence and an HF module for transmitting the data in the time slots.

An object of the present invention, therefore, is to provide a methodand system for logging on a mobile unit at a fixed station a mobile unitis enabled to be logged on at a fixed station for a data transmissionsystem, in which system data is transmitted in time slots on a number ofcarrier frequencies and the carrier frequency is changed from one timeslot to the next.

SUMMARY OF THE INVENTION

This object is achieved according to the present invention by a methodfor logging on a mobile unit at a fixed station for a transmission ofdata by radio, in which transmission the data is transmitted in timeslots on a number of carrier frequencies (TDMA system) and the carrierfrequency is changed, for example, from one time slot to the next timeslot in accordance with a predetermined sequence. According to thepresent invention, check data which indicates the position of thecarrier frequency of the current (instantaneously broadcast) time slotin the predetermined sequence is broadcast by the fixed station. Themobile unit can then determine the position of the carrier frequency ofthe current time slot in the predetermined sequence via the check data.The mobile unit to which the entire sequence is known can thendetermine, on the basis of the position of the carrier frequency in thepredetermined sequence, the carrier frequency which is to be changed tonext. As a result, synchronization of the change of the carrierfrequency of the mobile unit with that of the fixed station is achieved.

The check data can be transmitted only during a logging-on mode. Afterthe logging-on mode has been terminated, normal transmission of, forexample, voice information data between the mobile unit and the fixedstation then can take place.

The carrier frequency change can be carried out via a sequence selectedfrom a plurality of predetermined sequences. The check data then canindicate, beyond the position of the carrier frequency of the currenttime slot in the predetermined sequence, which of the number ofpredetermined sequences is selected and used. The predeterminedsequences can be determined, in particular an algorithm (hop algorithm).

It is possible to sense which of the number of carrier frequencies issubject to interference. During the logging on of the mobile unit at thefixed station, a carrier frequency which is prescribed by thepredetermined sequence also is used if this carrier frequency has beensensed as being subject to interference. After the logging on has beenconcluded, that carrier frequency of the predetermined sequence which issubject to interference is passed over during the normal transmission ofdata. This ensures that, during the logging-on mode, the carrierfrequency change prescribed by the predetermined sequence is strictlycarried out in order to ensure that the frequency of the mobile unit issynchronized with that of the fixed station in the sense of logging on.In particular, the so-called 2.4 GHz ISM frequency band can be used fortransmission. Also, the number of available carrier frequencies can beat least 75 and, in particular, 96.

In addition, according to the present invention a system for wire-freetransmission of data between a mobile unit and a fixed station isprovided. The fixed station here has an RF module for transmitting thedata in time slots on a number of carrier frequencies in the sense of atime division multiplex system. A device stores a predetermined sequencein order to define a change of the carrier frequency, for example fromone time slot to the next, and outputs this predetermined sequence tothe RF module. The data broadcast by the fixed station have check datawhich indicates the position of the carrier frequency of the currenttime slot in the predetermined sequence. The mobile unit has a devicefor determining the position of the carrier frequency of the currenttime slot in the predetermined sequence via the check data. In analternative embodiment, the check signal also can specify the carrierfrequency which the base station will “jump to” next.

In yet a further embodiment, the check data can specify which carrierfrequency the base station will use in the m-th time slot or m-th frame.This is advantageous if a mobile unit is in the so-called idle-locked ormultiframe mode. In such a mode, a mobile unit resynchronizes with thebase station only in every m-th time slot or frame if the mobile unit isnot in the process of active voice communication with the base station.

The check data does not have to be broadcast in every time slot orframe. If a mobile unit which would like to synchronize with a basestation receives a time slot or frame which does not contain check data,it scans all the carrier frequencies again. This procedure is repeateduntil the mobile unit receives from the base station a time slot orframe which contains the check data.

The fixed station can have a switching device for switching over betweena logging-on mode, in which a mobile unit, or a further mobile unit, canbe logged on at the fixed station, and a normal transmission mode fornormal transmission of information data. The check data is broadcastautomatically only if the switching device is switched to the logging-onmode. In the normal transmission mode, the check data is not broadcast,or is only broadcast on request.

A number of predetermined sequences can be provided in the outputdevice. The check data then has data which goes beyond the position dataand which indicates the sequence currently in use. In addition, theoutput device can have a processor which calculates the predeterminedsequence via an algorithm.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the Detailed Description of thePreferred Embodiments and the Drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system of the present invention for transmitting data ina wire-free fashion;

FIG. 2 shows a time frame of a data transmission standard such as isused in the present invention;

FIG. 3 shows a detailed illustration of a time frame for a carrierfrequency in accordance with the present invention; and

FIG. 4 shows a schematic representation of a frequency hopping spreadspectrum system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, the general design of the system according tothe present invention for radio transmission will be explained first. Asis generally known, a system for the transmission of data by radio has afixed station 1 and a plurality of mobile units (mobile stations,cable-free telephones) 2, 3, 11. The fixed station 1 is connected to thelandline network with a terminal line 10. The fixed station 1 has anantenna 6 via which it is possible to communicate, for example, with themobile unit 2 via a radio transmission path 8 or with the mobile unit 3via a radio transmission path 9. The mobile units 2, 3, 11 each have anantenna 7 for receiving and transmitting data.

The internal design of a fixed station 1, insofar as it is ofsignificance for the present invention, will now be explained in moredetail. A processor 15 which determines a predetermined sequence via apredetermined algorithm (hop algorithm) is provided in the fixed station1. As an alternative, a number of different algorithms may be providedin the processor 15 so that it can determine different sequences inaccordance with the respectively used algorithm. The sequencesdetermined by the processor 15 are then transmitted to a storage andoutput device 13. The storage and output device 13 transmits to an RFmodule 4 either the sequence which is continuously determined by theprocessor 15 or a sequence which has been previously permanently storedin it.

The RF module 4 receives and transmits data on a carrier frequency f_(x)which depends on the current value of the sequence transmitted from thestorage and output device 13. Therefore, a radio transmission takesplace on a carrier frequency f_(x) wherein the currently used carrierfrequency either is determined indirectly by the processor 15 via analgorithm or, alternatively, is determined directly from the value of asequence which has been permanently stored in the storage and outputdevice 13.

The internal design of a mobile radio unit, insofar as it is relevant tothe present invention, will now be described in more detail. In thisrespect, the design of a mobile radio unit 2, 3, 11 is substantiallysymmetrical to the internal design of the fixed station 1 describedabove. That is, each mobile radio unit 2, 3, 11 has, as illustrated inthe present invention only for the mobile radio units 2 and 11, aprocessor 16. This processor 16 determines, via either one or a numberof available hop algorithms, a sequence which it transmits to a storageand output device 12. The storage and output device 12 transmits to anRF module 5 either the values of the sequence based on the algorithmwhich are determined continuously by the processor 16 or, alternatively,the values of a sequence which has been permanently stored in it. The RFmodule 5 transmits or receives data on a carrier frequency f_(x) whoselevel depends on the value of the sequence transmitted to it by thestorage and output device 12. A mobile unit 2, 3, 11 therefore receivesor transmits data on a carrier frequency f_(x) whose level dependseither on the current value of the sequence determined by the processor16 or on the value of a sequence which has been permanently stored inthe storage and output device 12.

It is to be noted here that the processor 15 in the fixed station 1 andthe processors 16 in the mobile units 2, 3, 11 are based on the samealgorithm for determining sequences or, in the event that a number ofalgorithms are available, have the same selection of algorithms. In theevent that the sequence is not determined continuously by the processor15, 16 but rather permanently prescribed in the storage and outputdevices 12, 13, the sequence which is stored in the storage and outputdevice 13 of the fixed station 1 is, of course, identical to thesequences which are respectively stored in the storage and outputdevices 12 of the mobile units 2, 3, 11.

A transmission standard such as is used in the present invention willnow be explained with reference to FIG. 2. As is clear in FIG. 2, datais transmitted in chronological succession in a number of time slots (24time slots Zx in the case illustrated), using the time divisionmultiplex method TDMA (Time Division Multiple Access) on a number ofcarrier frequencies f_(x) (of which ten are illustrated). In FIG. 2,duplex mode is used on the carrier frequencies. This means that afterthe base station has transmitted the first twelve time slots Zx, itswitches to reception and it receives the second twelve time slots(13–24) in the opposing direction.

In the event that the so-called DECT Standard is used for transmission,the chronological duration of a time frame is 10 milliseconds, and 24time slots Zx are provided, namely twelve time slots for thetransmission from the fixed station to mobile units and a further twelvetime slots Zx for the transmission from the mobile units to the fixedstation. In the DECT Standard, 10 carrier frequencies f_(x) between 1.88GHz and 1.90 GHz are provided.

However, the present invention also is used for transmission in theso-called 2.4 GHz ISM (Industrial Scientific Medical) frequency band.The ISM frequency band has a bandwidth of 83.5 MHz. In accordance withthe specification “FCC Part 15” (Federal Communications Commission), atleast 75 carrier frequencies must be distributed over these 83.5 MHz.Distributing the 83.5 MHz bandwidth over 96 carrier frequencies, i.e. achannel spacing of 864 kHz, is particularly advantageous.

The abovementioned frequency bands and standards are mentioned purely byway of example. The only fundamental precondition for the presentinvention is that a so-called frequency hopping spread spectrum is used;i.e., a number of carrier frequencies are available and the carrierfrequency f_(x) selected for the transmission is changed from time totime. A precondition of such a change is that the data be transmitted intime slots Zx (time division multiplex method). The so-called DECTStandard as well as any other modified standard based on this DECTStandard, is therefore suitable. A modification can, in this respect,include a reduction (halving) in the number of time slots per frame, asa result of which the bit rate and, consequently, the necessary basicbandwidth of the transmission can be reduced (halved).

How the selection of a carrier frequency f_(x) for a specific time slotZx is carried out will now be explained with reference to FIG. 4. Itwill be assumed that, at the time of the time slot Z1, the processor 15of the fixed station 1 determines, on the basis of an algorithm, a valuewhich the RF module 4 of the fixed station 1 converts indirectly into acarrier frequency f₁. In FIG. 4, the hatching shows that the carrierfrequency f₁ is selected at the time of the time slot Z1. At thetransition from the time slot Z1 to the following time slot Z2, thecarrier frequency f_(x) is inevitably changed. As is illustrated by anarrow in FIG. 4, it is possible, for example, for the processor 15 ofthe fixed station 1 to determine via its algorithm a value which isconverted by the RF module 4 into a carrier frequency f₃. In the sameway, a carrier frequency f₂ then can be selected for the time slot Z3,which is illustrated by hatching and by an arrow.

In the example above, the case was explained in which the carrierfrequency is changed after a time slot in each case. However, for thepresent invention it is only significant that the change of the carrierfrequency takes place in each case after a predetermined time period.This also may be, for example, a frame.

The fixed station 1, therefore, changes the carrier frequency f_(x) fromthe carrier frequency f₁ to the carrier frequency f₃ and then to thecarrier frequency f₂ on the basis of the sequence determined by theprocessor 15. If communication is to take place between the fixedstation 1 and a mobile unit 11, it is necessary to ensure that themobile unit 11 can follow synchronously the sequence of carrierfrequency f_(x) changes carried out by the fixed station 1. This is aparticular problem when a mobile unit 11 first is to be integrated intoa radio transmission system; i.e., has to be logged on and signed on atthe fixed station 1. During unsynchronized operation of the new mobileunit 11 after it has been switched on, the mobile unit 11 will changethe carrier frequencies f_(x) used in the way prescribed by itssequence. The sequence as such is identical here with the sequence 1,which is prescribed in the fixed station 1 and explained above. However,this does not ensure that the sequence of the mobile unit 11 issynchronized with the sequence of the fixed station 1 after the mobileunit 11 has been switched on.

FIG. 3 illustrates how it is ensured according to the present inventionthat the new mobile unit 11 carries out carrier frequency changes whichare synchronous with the fixed station 1. As is clear in FIG. 3, thedata transmitted in a time slot (channel) Zx is, for the most part,information data; i.e., for example, data which represents an item ofvoice information of a telephone call. Before the range of theinformation data there is then a check range which is referred to as Afield in the DECT Standard. In this check range, data is provided forsynchronizing the operation of a mobile unit 11 to be logged on with theoperation of the fixed station 1. If a number of algorithms areavailable to the processor 15 in the fixed station 1 for determining thesequence which directly prescribes the changes of the carrier frequencyf_(x) of the fixed station 1, the check range contains data whichidentifies the algorithm currently in use. Further synchronization datacontained in the check range is data which indicates which position inthe predetermined sequence corresponds to the carrier frequency f_(x)used for the current time slot Zx. The data of the check range which isillustrated in FIG. 3, namely data which refers to the algorithm usedand which refers to the current position of the sequence of the currentalgorithm, is broadcast by the fixed station 1 to the mobile unit 11. Asan alternative, the check signal also can specify the carrier frequencywhich the base station will “jump to” next.

As a further alternative, the check data can specify which carrierfrequency the base station will use in the m-th time slot or m-th frame.This is advantageous if a mobile unit is in the so-called idle-locked ormultiframe mode. In such a mode, a mobile unit resynchronizes with thebase station only in every m-th time slot or frame if said mobile unitis not in the process of active voice communication with the basestation.

The check data does not have to be broadcast in every time slot orframe. If a mobile unit which would like to synchronize with a basestation receives a time slot or frame which does not contain check data,it scans all the carrier frequencies again. This procedure is thenrepeated until the mobile unit receives from the base station a timeslot or frame which contains the check data.

After it has been switched on, the mobile unit 11 scans the availablerange of carrier frequencies f_(x) until it senses the carrier frequencyf_(x) currently being used by the fixed station 1. During this sensingof the carrier frequency f_(x) currently in use, the mobile unit 11 alsosenses the data of the check range of the data broadcast by the fixedstation 1. At first, the mobile unit 11 can determine which algorithm iscurrently being used by the processor 15 in the fixed station 1. Thisalgorithm, of course, indirectly prescribes the charging of the carrierfrequencies of the fixed station 1.

In addition, the mobile unit 11 can sense, from the position data of thecheck range, which position in the predetermined frequency correspondsto the broadcast carrier frequency. The mobile unit 11 then is,therefore, aware of the algorithm in use and of the position in thesequence. The mobile unit 11 can then determine independently via theposition in the sequence, which is known here, as well as the sequencestored in it, which carrier frequency f_(x) will be used by the fixedstation 1 in the following time slot Z_(x). From the information fed toit, the mobile unit 11 can generate information for the carrierfrequencies to be used in the following time slots Z_(x). Thus, it ispossible to communicate with the fixed station 1 as is necessary for asigning-on or logging-on procedure. As a result of the informationsupplied relating to the future carrier frequency change, the mobileunit 11, is therefore, synchronized with the fixed station 1.

The fixed station 1 can have a switching device 14 which can be switchedbetween two positions; namely, a position in the logging-on mode R and aposition corresponding to the normal transmission mode. Only if theswitching device 14 is switched to logging-on mode R does the fixedstation 1 automatically broadcast the check range data necessary forsynchronization with a mobile unit to be newly logged on. This data isnamely, the information relating to the algorithm in use and theinformation relating to the position in the predetermined frequency onthe basis of the algorithm. If the switching device 14 is switched tothe normal transmission mode N, the aforesaid synchronization data arenormally not broadcast; that is, only broadcast on request from a mobileunit.

A problem when logging on a further mobile unit 11 can result from aso-called noise source fall-back mode. First, it will be explained whataction the fixed station 1 takes in accordance with this noise sourcefall-back mode with regard to the carrier frequency selection. Withreference to FIG. 4, it is clear that at the time of the time slot Z3the carrier frequency f₂ is indicated by the predetermined frequency. Itnow will be assumed that the predetermined sequence for the time of thetime slot Z4 indicates a change to the carrier frequency f₄. Inaddition, it will be assumed that, for example in the preceding timeframe of the transmission, the fixed station 1 has determined thatinterference occurred during a transmission on the carrier frequency f₄.This interference may result, for example, from the fact that anotherradio transmission system is adversely affecting this carrier frequencyf₄. If the fixed station 1 is in the so-called noise source fall-backmode, when selecting the carrier frequency f_(x) for the time slot Z4 itwill not select the carrier frequency f₄ which is, of course, actuallyprescribed by the predetermined frequency. The carrier frequency f₄which is sensed as being subject to interference is instead passed overand another carrier frequency f_(x), for example the carrier frequencyf_(x) which follows in the predetermined frequency, is selected for thetime slot Z4 (as illustrated by the arrow P₁). In the case illustratedin FIG. 4, the carrier frequency selected for the time slot Z4 is not,therefore, the carrier frequency f₄ which is sensed as being subject tointerference but is instead the carrier frequency f₁ which is sensed asbeing free of interference.

Even if this noise source fall-back mode has, great advantages duringthe radio transmission mode with mobile units 2, 3 which already havebeen integrated, it is clear that this noise source fall-back modesimultaneously causes large problems for the logging on of a new mobileunit 11. The mobile unit 11 will, in fact, determine, on the basis ofthe algorithm stored in it and the position of the carrier frequencywhich is known to it from the check range of the data transmitted fromthe fixed station, in the predetermined sequence in accordance with thealgorithm at the time of the time slot Z3, that a transmission on thecarrier frequency f₄ will take place starting from the next value of thesequence at the time of the time slot Z4. However, if, owing to thenoise source fall-back mode, the fixed station 1 selects the carrierfrequency f₁ at the time of the time slot Z4 in order to avoid thecarrier frequency f₄ which is subject to interference, and at the sametime the mobile unit 11 to be logged on selects, on the basis of theinformation available to it, the carrier frequency f₄ at the time of thetime slot Z4, synchronization of the operation of the fixed station 1with that of the mobile unit 11 fails. If, for this reason, thelogging-on mode R is selected by the switching device 14 in the fixedstation 1, the noise source fall-back mode of the fixed station 1 issimultaneously switched off. This means that, in contrast with thenormal mode in which the fixed station 1 will, in order to avoid thecarrier frequency f₄ which has been recognized as being subject tointerference, switch, in a position of the switching device 14 inlogging-on mode R, to the carrier frequency f₄ at the time of the timeslot Z4 as is prescribed by the sequence on the basis of the algorithmof the processor 15. This is so even though the fixed station 1 is awarethat the carrier frequency f₄ is subject to interference. The change ofthe carrier frequency f_(x) from time slot Z3 to time slot Z4 isillustrated in FIG. 4 by the unbroken arrow P₂. As a result of the factthat the noise source fall-back mode of the fixed station 1 issimultaneously switched off when the switching device 14 is positionedin logging-on mode R, it is therefore ensured that a synchronization ofthe operation of the mobile unit 11 with that of the fixed station 1 cantake place. After the signing-on procedure or logging on of the mobileunit 11 at the fixed station 1 has been completed, the switching device14 is then switched back from the logging-on mode R to the normaltransmission mode N, which can take place in an automated way, and thenoise source fall-back mode can be switched on again automatically.

However, the noise source fall-back mode also can remain switched onduring the signing-on procedure. In this context, it is to be notedthat, in accordance with the exemplary embodiment, 96 carrierfrequencies are provided, of which a maximum of 21 can be locked out inorder to avoid impinging upon the US-American Specification “FCC part15”. Therefore, the mobile unit knows the majority of carrierfrequencies used, even in the noise source fall-back mode. Thus, ifcommunication does not come about between the mobile unit and the fixedstation in a frame owing to a frequency lock-out which is not known tothe mobile unit, in all probability it will be possible to resumecommunication in the next frame with a new carrier frequency.

Therefore, according to the present invention, a method and a device forensuring synchronism during the initial logging on of a new mobile unitat a fixed station is provided with a so-called frequency hopping spreadspectrum system on a time division multiplex basis.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the invention as set forth in the hereafter appended claims.

1. A method for logging-on a mobile unit at a fixed station, the methodcomprising the steps of: selecting, at the fixed station, a sequence inwhich a plurality of carrier frequencies are changed from one timeperiod to the next when communicating with the fixed station;transmitting data from the fixed station, wherein the data comprisescheck data, said check data comprising identification of the selectedsequence and a time slot position of a specific carrier frequency beingused by the fixed station during the step of transmitting; determining,via the mobile unit, the specific carrier frequency being used by thefixed station when the data was transmitted; determining, via the mobileunit, the selected sequence in which the plurality of carrierfrequencies are changed from one time period to the next, using thetransmitted check data; determining, via the mobile unit, the time slotposition of the specific carrier frequency relative to the selectedsequence, using the transmitted check data; determining, via the mobileunit, a subsequent carrier frequency used in a subsequent time periodrelative to the time slot position of the specific carrier frequency,using the check data; and changing to the subsequent carrier frequency,via both the mobile unit and the fixed station, after a predeterminedtime period in accordance with the selected sequence.
 2. A method forlogging-on a mobile unit at a fixed as claimed in claim 1, the methodfurther comprising the step of: automatically transmitting the checkdata during a logging-on mode between the mobile unit and the fixedstation.
 3. A method for logging-on a mobile unit at a fixed as claimedin claim 1, wherein the fixed station and mobile unit include aplurality of different sequences in which the plurality of carrierfrequencies are changed from one time period to the next.
 4. A methodfor logging-on a mobile unit at a fixed station as claimed in claim 1,wherein the set of selecting, at the fixed station, a sequence in whichone or more carrier frequencies are changed from one time slot to thenext is determined via an algorithm.
 5. A method for logging-on a mobileunit at a fixed station as claimed in claim 1, the method furthercomprising the steps of: sensing which of the plurality of carrierfrequencies is subject to interference; using, during the logging-on ofthe mobile unit, a carrier frequency which is prescribed by thepredetermined sequence and has been sensed as being subject tointerference; and passing over the carrier frequency sensed as beingsubject to interference after logging-on has been completed.
 6. A methodfor logging-on a mobile unit at a fixed station for a transmission ofdata by radio as claimed in claim 1, wherein a 2.4 GHz ISM frequencyband is used for transmission.
 7. A method for logging-on a mobile unitat a fixed station for a transmission of data by radio as claimed inclaim 1, wherein the number of available carrier frequencies is at least75.
 8. A method for logging-on a mobile unit at a fixed station for atransmission of data by radio as claimed in claim 1, wherein the numberof available carrier frequencies is at least
 96. 9. A system for thetransmission of data between a mobile unit and a fixed station, thesystem comprising: a fixed station, the fixed station comprising: aprocessor that selects a sequence in which a plurality of carrierfrequencies are changed from one time period to the next whencommunicating with the fixed station; and a transmitting module thattransmits data from the fixed station, wherein the data comprises checkdata, said check data comprising identification of the sequence selectedby the processor, and a time slot position of a specific carrierfrequency being used by the fixed station during transmission; a mobileunit, wherein the mobile unit comprises a processor that determines thespecific carrier frequency being used by the fixed station when the datawas transmitted, the selected sequence in which one or more carrierfrequencies are changed from one time period to the next, using thetransmitted check data, the time slot position of the specific carrierfrequency relative to the selected sequence, using the transmitted checkdata, and a subsequent carrier frequency used in subsequent time periodrelative to the time slot position of the specific carrier frequencyusing the check data; and sequencing means for changing to thesubsequent carrier frequency in both the mobile unit and the fixedstation after a predetermined time period in accordance with theselected sequence.
 10. A system for the transmission of data between amobile unit and a fixed station as claimed in claim 9, the systemfurther comprising: a switching device in the fixed station forswitching over between a logging-on mode and a normal transmission mode,wherein the check data is transmitted automatically if the switchingdevice is switched to the logging-on mode.
 11. A system for thetransmission of data between a mobile unit and a fixed station asclaimed in claim 9, wherein each of the fixed station and the mobileunit respectively includes a plurality of different sequences in which aplurality of carrier frequencies are changed from one time slot to thenext.
 12. A system for the transmission of data between a mobile unitand a fixed station as claimed in claim 9, wherein the processor in eachof the fixed station and the mobile unit process the selection of thesequence via an algorithm.
 13. system for the transmission of databetween a mobile unit and a fixed station as claimed in claim 9, whereinthe plurality of carrier frequencies lie in a 2.4 GHz ISM radio band.